Time-domain spectroscopy using, for example, terahertz radiation is known in the art, wherein in a first step the time dependence of a terahertz pulse (i.e. the pulse shape) is determined, i.e. the terahertz pulse is traced in the time domain. Subsequently, a frequency spectrum is generated by applying a Fourier transformation to the determined pulse shape. In order to determine the pulse shape, the terahertz pulse is sampled, for example, using an optical delay line. For example, the optical delay line comprises a step motor for discretely moving an optical element and thus successively changing the optical path length (i.e. the delay time) provided by the delay line. A position sensor is used to measure the actual position of the optical element with μm precision to determine the corresponding delay time on a femtosecond scale. Further, the amplitude of the terahertz pulse is measured step-by-step at a plurality of positions of the optical element using lock-in technique which results in a rather long measurement time.
Faster data acquisition can be realized by generating a continuous periodic movement of the optical element of the delay line, wherein the actual position of the optical element is also measured by means of a position sensor. However, the precision of the measurement of the position of the continuously moving optical element (i.e. of the delay time) appears to be more limited than for measurements with stopped delay lines.